Conventional methods of making ductile intermetallic-bonded ceramic composites involve hot-pressing of powder mixtures in graphite dies. Complex shaped articles cannot generally be made via such methods because of the axial nature thereof.
For such materials to become viable commercial products, development of sintering techniques not involving the application of mechanical pressure was required. A method was developed to sinter these types of composites to densities to about 90% theoretical density (T.D.) without the application of mechanical pressure.
Attempts to sinter intermetallic-bonded ceramic composites utilizing the same types of powders used for the hot-pressing of these materials were unsuccessful. The relative particle size of the Ni.sub.3 Al powders was significantly larger than the ceramic powders. In addition, the Ni.sub.3 Al powders employed were pre-alloyed with boron and were characterized by ductility and high strength. Normally, powder mixtures are dispersed by milling together, however, because of the large size difference and the inherent ductility of the Ni.sub.3 Al powders, milling was not sufficiently effective in producing a homogeneous mixture. The poor densification (&lt;90% T. D.) is attributed to the insubstantial wetting behavior between the large Ni.sub.3 Al particles and the smaller WC and TiC. In addition, the sintered composites revealed large pores believed to be due to the void left by the original Ni.sub.3 Al particle after it was `wicked` into the surrounding carbide particles.
For further information, please refer to the following:
1. U.S. Pat. No. 4,762,558 issued to German, et al. teaches the use of elemental Ni and Al powders in a reaction sintering method of making Ni.sub.3 Al at low temperatures (&lt;1000.degree. C.). No additions of ceramic powders are used; Ni.sub.3 Al content is 100%.
2. U.S. Pat. No. 4,919,718 issued to Tiegs, et al. teaches the use of large-particle-size (&gt;20 .mu.m) pre-alloyed ductile Ni.sub.3 Al powders to make composites that are densified by hot-pressing. The content of Ni.sub.3 Al ranges from 5-20 wt. %.
3. U.S. Pat. No. 5,271,758 issued to Buljan, et al. teaches the use of elemental Ni and Al powders with Al.sub.2 O.sub.3 --TiC mixtures that are densified by hot-pressing. The content of Ni.sub.3 Al ranges from 5-20 wt. %.
4. Mei, et al, "Investigation of Ni.sub.3 Al-Matrix Composites Strengthened by TiC", J. Mater. Res., Vol. 8, No. 11, Mater. res, Soc. (1993) teaches the use of an in-situ reaction of Ni, Al, C and Ti powders to produce materials with 35 wt. % (45 vol. %) TiC in a Ni.sub.3 Al matrix (55 vol. %).